This won't be an issue in the new building for people on WiFi (I'll be setting it up with WPA2).
It will be an issue because they'll have the keys. It isn't quite as simple to do but its possible to do. Encryption on WiFi just stops outsiders from accessing your network.
But wouldn't people on the wired LAN be vulnerable to this until these services start finally using https?
Normally you don't get packets for other machines on the network. The switch just sends you packets for your own computer however you can force it to send you everything or to start masquerading as the gateway.